As is known, in the aircraft industry, reducing the noise level inside the cabin is now a major design issue.
Outside noise normally comes from the engines, the moving parts associated with the component parts of the aircraft, and from airflow over the airframe, and propagates from the fuselage to the interior of the aircraft mainly along air and structural paths, i.e. through structural connecting points between the fuselage and the inner fuselage panels.
Most aircraft, and particularly helicopters, are built about a supporting frame. More specifically, an outer covering is applied to the frame; thermal and acoustic insulating material is applied between the frame and the cabin; and the frame and insulating material are covered with trim panels on the side facing inwards of the cabin.
Soundproofing inside the cabin thus depends on the trim panels, the type of acoustic insulating material used, and the cabin furnishings (seats, carpeting, etc.).
Number 1 in the exploded cross section in FIG. 1 indicates as a whole one example of a known trim panel, which has a multilayer structure and substantially comprises:                an outer layer 2 positioned in use facing the fuselage and made of composite material of carbon or glass fibres impregnated with epoxy resin;        a honeycomb structural layer 3 positioned contacting layer 2 and made of composite material containing, for example, metha-aramide (trade name) NOMEX®) fibres;        an intermediate layer 4 with the same composition as layer 2 and positioned contacting layer 3, on the opposite side to layer 2;        a trim layer 5 defining the side of panel 1 facing inwards of the cabin and typically made of leather; and        a layer 6 of porous, acoustic energy dissipating material, typically closed-call, soft-touch foam, interposed between layers 4 and 5.        
Known panels with the above structure have the drawback of fully reflecting soundwaves on both sides, i.e. also on the side facing inwards of the cabin, which is normally impervious to air, so that any acoustic energy propagating inside the cabin cannot be dissipated.